This laboratory has been trying to determine the structure of retrovirus cores by cryoEM. They have been finding them to be somewhat heterogeneous and not icosahedral. They sent a sample to the STEM for mass analysis. It was very heterogeneous, with filled and empty capsids of different sizes, plus some tubes and sheets. Possibly a different preparation would be better. A new sample was more pure, but still appeared to have heterogeneous cores.